A curable composition containing an epoxy resin and a cyanic acid ester has an excellent electrical performance together with a high heat resistance and superior adhesive properties. Therefore, it is used for various kinds of members in the electrical or electronic field such as a printed wiring board and an insulating sealing material for a semiconductor.
In addition, in cases where it is used as a sealing material for a power semiconductor and a LED, an adhesive agent and an adhesive base material, a resin composition further containing a filler component and a resin sheet thereof, which have heat diffusion characteristics, are used.
For example, there were disclosed an epoxy resin composition, containing a polyfunctional epoxy compound such as trifunctional epoxy or tetrafunctional epoxy aromatic compound, a phenol-modified polyphenylene ether, a cyanate compound and a curing accelerator as an essential component, and has a high heat resistance, a low dielectric tangent and excellent adhesive properties (Patent document 1); and a prepreg which contains a cyanate resin, an epoxy resin, an imidazole compound and an inorganic filler as an essential component, and has a low dielectric constant, excellent laser machining properties and a high flame retardancy (Patent document 2).
Furthermore, there were disclosed a resin composition containing a cyanate resin and/or a prepolymer thereof, an epoxy resin having no halogen atoms, an imidazole compound and an inorganic filler as an essential component, and has a flame retardancy and a high heat resistance along with a low expansion coefficient (Patent document 3); a resin composition for a soldar resist containing a cyanate resin and/or a prepolymer thereof, an epoxy resin not having no halogen atoms, a phenoxy resin, an imidazole compound and an inorganic filler, and has a low linear expansion coefficient, a high heat resistance, an excellent thermal shock resistance and a high moisture resistance (Patent document 4); and a resin composition containing an epoxy resin, a cyanate ester resin, an adduct of an imidazole compound with an epoxy resin, and a metallic curing catalyst as an essential component, and has a low coefficient of thermal expansion, and can form a uniform roughened surface having a low roughness degree and also has excellent adhesive properties to a conductive layer formed on the said roughened surface (Patent document 5).
Moreover, there was disclosed an epoxy resin composition having a high flame retardancy, a high solder crack resistance and a superior in fluidity as well as little warpage, containing at least one epoxy resin selected from the trifunctional epoxy resins and the tetrafunctional epoxy resins, a curing agent having at least two groups which react with an epoxy group within a molecule, a compound having at least two cyanato groups within a molecule and an inorganic filler, as an essential component (Patent document 6).
In addition, there were disclosed an insulating sheet containing a polymer having 10,000 or more of weight-average molecular weight, a curable compound having an epoxy group or an oxetane group, a cyanate compound having 50 to 200 of cyanate equivalent, a curing agent and a filler, and has high handling properties in the uncured state and can obtain a cured material having a low relative dielectric constant (Patent document 7); and a solventless one liquid type cyanic acid-epoxy composite resin composition containing a polyfunctional cyanic acid ester having 2.5 or more of average number of cyanato group or a mixture thereof, a liquid polyfunctional epoxy resin having 2.5 or more of average number of epoxy group or a mixture thereof, and an amine latent curing agent, and has a curability, a heat resistance, a low permittivity and a low dielectric tangent (Patent document 8).
However, these prior arts could not satisfy sufficiently all characteristics required in the coating process or sheet forming process, such as coating properties, a flexibility, and a heat resistance of a cured material.